This invention is primarily directed towards improvements in a liquid feed apparatus for use in liquid chromatography systems. However, the pumping unit and various aspects thereof are useful wherever a precise, controlled movement of liquid is desired and, notably, in the movement of physiological liquids in the medical field and movement of fluids in analytical processes other than chromatography. Moreover, the various aspects of the invention include specific improvements in the field of controlling a pump drive means, measuring liquid pressure, and pump design.
Because the invention was made with the immediate aim of solving problems relating to liquid delivery systems used in liquid chromatography systems, the background relevant to such problems is set forth hereinbelow:
Liquid chromatography is a system whereby a mobile liquid phase is passed through a compartment containing a stationary phase. During the passage of the liquid phase, it interacts physically or chemically with the stationary phase. This interaction results in a separation of components in the liquid phase. Such a separation is usually manifested by the fact that different components in the liquid phase pass through the stationary phase at different rates. An analysis of samples of the effluent from the compartment taken over a period of time provides a basis for determing the chemical composition of the input liquid.
The analysis of the efflux is usually made continuously with a refractometer or the like.
It is desirable to supply the liquid to the aforesaid chromatography compartment (usually an elongate column packed with particles) at very high pressures, usually from 100 to 10,000 psig.
A number of other attributes are desirable for pumping systems used in chromatography:
They should have a wide operating range in terms of pump throughout capacity.
They should have a minimal temperature deviation from the temperature of the fluid being processed. Thus a motor integral with the pumping system should be either very well insulated from the liquid being pumped or should run at a temperature close to the temperature of the liquid.
There should be a minimum flush time in which one liquid being pumped can be replaced by another liquid being pumped.
The pumping system should be capable of providing means to moderate the pump output capacity in response to pump output pressure, and this moderating means should be sensitive enough to allow compensation for compressibility on the pump output side of the system, whether this compressibility be a characteristic of fluid being pumped or of the pumping system itself.
The pumping system should include protective means to shut down the pump in an emergency, i.e. when the outlet pressure reaches a certain level.
The pump should provide as little mixing of the liquid being pumped as is possible and preferably so little mixing that recycling of the liquid through the pump is feasible.
The pumping action should be steady, i.e. pulseless.
In general the above-listed attributes tended to be self-conflicting when a chromatography system was operated according to the methods of the prior art. For example, the use of pulse dampeners tended to result in undesirable mixing of the fluid. Yet pulse-imparting pumps were those most acceptable for reaching the desired high feed pressure. And motors used to drive said pumps generated considerable heat -- enough to affect the characteristics of the fluids being pumped when the motor was packaged proximate the pump as is convenient and customary. Moreover, conventional pressure sensing means required dead space in which liquid would become relatively stagnant and from which it could not be quickly flushed when one wished to change the liquid being pumped.
Applicants therefore set as their objectives to provide liquid delivery apparatus and processes having a number of the above-listed attributes and overcoming many of those problems relating to conflicting design requirements.